Pressure gauges enjoy very extensive commercial and industrial and are consequently regarded as high production items. Because of such wide use, they are supplied by a plurality of manufacturers and sold in very price conscious competition. Each manufacturer instinctively strives to reduce product costs and improvements, however marginal, which reduce labor and/or materials that can contribute to cost savings in the end product.
Commonly affording pressure sensitivity in a pressure gauge is a Bourdon tube of a pressure-tight construction having a free end displaceably movable in response to pressure changes supplied at its inlet. To translate tube movement into values of pressure, a pointer is connected to the free end of the Bourdon tube which then moves with the tube relative to a calibrated dial plate. For insuring readout accuracy, provision is usually made in the course of assembly, for adjustably positioning the pointer or dial plate so that they coincide at the zero position of the dial.
In a conventional construction, the inlet end of the Bourdon tube is secured to either the casing or the stem, while the dial plate can be arcuately displaced relative to the pointer to effect the zero adjustment. Such construction is most suitable where ultimate mounting of the gauge affords flexibility in orientation for convenient dial viewing in-place on the equipment. In some instances, however, such flexibility is lacking as, for example, where the gauge casing is required to be keyed to the equipment in a registered orientation as is fairly common for those gauges installed on fire extinguishers. It can be readily appreciated that in constructions of this type, i. e. where keyed registration is required, it is not possible to pre-key the gauge prior to zero calibration if indeed orientation of the displaceable dial plate is to be maintained with respect to the registered setting afforded by the key. One approach toward resolving this problem has been to utilize an unsecured Bourdon tube supporting the pointer such that the pointer instead of the dial plate can be arcuately displaced as disclosed, for example, in U.S. Pat. No. 3,952,598. Despite recognition of the problem of keying the gauge with a secured Bourdon tube, a ready solution thereto has not heretofore been known.